This invention relates to a full color, xerographic printing system using a Raster Output Scanning (ROS) system incorporating a two wavelength (.lambda.) laser diode source for the ROS and a charge retentive surface in the form of a belt or drum structure which is responsive to the two wavelengths and, more particularly, a red, green, and blue (RGB) plus black (K) imaging system which minimizes image desaturation and which can be selectively utilized for creating perfectly registered RGB process color images or extended gamut process color using black toner in combination with the RGB colors or K+2 (black+2 colors) color images where the colors may comprise highlight and/or logo colors.
Additive p color imaging using RGB color toners requires exact registration of images. Misregistered images or image overlap of RGB color toners results in a dirty brown color which desaturates the colors. This is because RGB colors each absorb two thirds of the visible spectrum resulting in undesirable browns when overlap occurs. In contrast, cyan, magenta, and yellow (CMY) colors absorb only a third of the visible spectrum. Overlaps lead to RGB colors.
Xerocolography (dry color printing) is a color printing architecture which combines multi-color xerographic development with multiwavelength laser diode light sources, with a single polygon, single optics ROS and with apolychromatic, multilayered photoreceptor to provide color printing in either a single pass or in two passes. In a single pass imaging machine, an image is formed by passing portion of the image receiving member past the processing stations only once. Inherently perfect registration is achieved since the various color images are all written at the same imaging station with the same ROS. In all, three perfectly registered latent images are written in this manner. Two of the three images are immediately developable because their voltage levels are offset from a background level while the voltage level of the third image is at the time of its creation equal to the background voltage level. An electrostatically distinguishable third image is formed when the photoreceptor is exposed to flood illumination of a predetermined wavelength.
It is desirable to provide as many features in a single imaging apparatus as possible. One is to create perfectly registered process color images using the additive primary colors red, green and blue. It is also desirable to be able to achieve the full gamut of the RGB images with a minimum of desaturation, and to extend the gamut further with black or other colors. Another desirable feature is being able to create K+2 colors where the colors are highlight colors and/or logo colors.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, are intended to provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 4,731,634 entitled "Apparatus for printing black and plural highlight color images in a single pass" granted to Howard M. Stark on Mar. 15, 1988 discloses a method and apparatus for rendering latent electrostatic images visible using multiple colors of dry toner or developer and more particularly to printing toner images in black and at least two highlighting colors in a single pass of the imaging surface through the processing areas of the printing apparatus. Two of the toners are attracted to only one charge level on a charge retentive surface to thereby providing black and one highlight color while two toners are attracted to another charge level to form the second highlight color.
U.S. Pat. No. 4,868,611 entitled "Tri-Level Xerography Scorotron Neutralization Concept" granted to Richard P. Germain on Sep. 19, 1989 discloses the use of a scorotron after the development of a first image. The scorotron serves to bring that first image to complete charge neutralization which removes the voltage responsible for the fringe fields thereby precluding fringe field development during the development of a subsequent image.
U.S. Pat. No. 5,049,949 entitled "Extension of tri-level xerography to black plus 2 colors" granted to Parker et al on Sep. 17, 1991 discloses a highlight color printing apparatus and method for forming one black and two color images. A tri-level image containing CAD (charged area development) and DAD (discharged area development) image areas and a background area is formed. A second DAD image is formed by discharging the background area forming part of the tri-level image.
U.S. Pat. No. 5,155,541 entitled "Single pass digital printer with black, white and 2-color capability" granted to Robert P. Loce et al on Oct. 13, 1992 discloses a method and apparatus for printing toner images in black and at least two highlighting colors in a single pass of the imaging surface through the processing areas of the printing apparatus. Imaging and development techniques of color photography and tri-level xerography are combined to produce images with black and two colors wherein the two highlighting colors are developed with only one color toner. A single imaging step forms a four level charge pattern on a charge retentive surface followed by development of two of the image levels using tri-level imaging techniques. Uniform exposure of the imaging surface, similar to that used in color photography techniques precedes development of the last image. The uniform exposure modifies the last developed image level and the background charge level allowing development of the last image with a single toner.
U.S. Pat. No. 5,221,954 entitled "Single pass full color printing system using a quad-level xerographic unit" granted to Ellis D. Harris on Jun. 22, 1993 discloses a four color toner single pass color printing system consisting generally of a raster output scanner (ROS) optical system and a quad-level xerographic unit and a tri-level xerographic unit in tandem. The resulting color printing system would be able to produce pixels of black and white and all six primary colors. The color printing system uses a black toner and toners of the three subtractive primary colors or just toners of the three subtractive primary colors.
U.S. Pat. No. 5,223,906 entitled "Four color toner single pass color printing system using two tri-level xerographic units" granted to Ellis D. Harris on Jun. 29, 1993 discloses a four color toner single pass color printing system consisting generally of a raster output scanner (ROS) optical system and two tri-level xerographic units in tandem. Only two of the three subtractive primary colors of cyan, magenta and yellow are available for toner dot upon toner dot to combine to produce the additive primary colors. The resulting color printing system would be able to produce pixels of black and white and five of the six primary colors, with pixel next to pixel printing producing all but the strongest saturation of the sixth primary color, an additive primary color. The color printing system uses either four color toners or a black toner and three color toners.
U.S. Pat. No. 5,534,990 entitled "Full color printing system using a penta-level xerographic unit" granted on Jul. 9, 1996 to Ellis D. Harris discloses a single pass full color printing system consisting generally of a raster output scanner (ROS) optical system and a quad-level xerographic unit and a penta-level xerographic unit in tandem. This full color printing system produces pixels of black and white and all six primary colors without toner upon toner.
U.S. Pat. No. 5,337,136 entitled "Tandem Tri-level Process Color Printer" granted to John F. Knapp et al on Aug. 9, 1994 discloses a tandem tri-level architecture. Three tri-level engines are arranged in a tandem configuration. Each engine uses one of the three primary colors plus one other color. Spot by spot, two color tri-level images can be created by each of the engines. The spot by spot images are transferred to an intermediate belt member, either in a spot on spot manner for forming full color images or in a spot next to spot manner to form highlight and/or logo color images. The images created by the tri-level engines can also be transferred to the intermediate in a manner such that both spot next to spot and spot on spot transfer is effected.
U.S. Pat. No. 5,347,303 entitled "Full Color Xerographic Printing System With Dual Wavelength, Single Optical System ROS And Dual Layer Photoreceptor" granted on Sep. 13, 1994 to Kovacs et al discloses a full color xerographic printing system, either two pass or single pass, with a single polygon, single optical system Raster Output Scanning (ROS) system which has a dual wavelength laser diode source for the ROS which images the dual beams at a single station as closely spaced spots or at two stations as closely spaced spots on a dual layer photoreceptor with each photoreceptor layer sensitive to or accessible by only one of the two wavelengths.
U.S. Pat. No. 5,373,313 entitled "Color xerographic printing system with multiple wavelength, single optical system ROS and multiple layer photoreceptor" granted to Gregory J. Kovacs on Dec. 13, 1994 discloses single pass color xerographic printing system with a single polygon, a single optical system Raster Output Scanning (ROS) system which has a multiple wavelength laser diode source for the ROS which images the multiple beams at a single station as closely spaced spots on a multiple layer photoreceptor with each photoreceptor layer sensitive to or accessible to only one of the multiple wavelengths.
U.S. Pat. No. 5,444,463 entitled "Color xerographic printing system with dual wavelength, single optical system ROS and dual layer photoreceptor" granted to Kovacs et al on Aug. 22, 1995 discloses a single pass color xerographic printing system with a single polygon, single optical system Raster Output Scanning (ROS) system which has a dual wavelength laser diode source for the ROS which images the dual beams at a single station as closely spaced spots on a dual layer photoreceptor with each photoreceptor layer sensitive to or accessible by only one of the two wavelengths.
U.S. Pat. No. 5,565,974 entitled "Penta-level xerographic unit" granted to Ellis D. Harris on Oct. 15, 1996 discloses a penta-level xerographic unit which produces five exposure levels on a photoreceptor. The five exposure levels select between a subtractive and an adjacent additive primary color in both the CAD and DAD operational regimes of a xerographic process. Exposure levels intermediate between the CAD and the DAD result in white. The selection of two possible colors in CAD, or two possible colors in DAD, or the selection of no toner yields a possibility of five colors. This penta-level xerographic unit can be used for a K+3 reduced color gamut printer, typically cyan, yellow and red plus black.
U.S. Pat. No. 5,592,281 entitled "Development scheme for three color highlight color trilevel xerography" granted to Parker et al on Jan. 7, 1997 discloses the creation of multiple color images in a single pass utilizing a multilayered photoreceptor structure having layers which are responsive to different wavelength lasers. A composite image including three images areas is formed with substantially perfect registration. A CAD and DAD image are developed using CMB (conductive magnetic brush) development and a second DAD image is developed using a non-interactive development system. Development of the second DAD image without developing halos around the CAD image is accomplished by uniformly recharging the photoreceptor to the background potential prior to the formation and development of the second DAD image.